Existing bush cutter blades are broadly classified into resin string blades and metal blades. The specialty fields of the two types are divided distinctly with each type having its own strong points. Specifically, the resin string blades provide a high level of safety and can cut grass under almost any condition. Moreover, the resin string blades can cut grass to the very limit, producing a neat, clean cut. Also, the cut grass is reduced to chips, and thus there is no need for cleaning. In particular, the resin string blades can safely cut grass around obstacles, grass containing foreign matter, or grass in gravelly places, which is something the metal blades are not good at. However, the resin string blades have the drawback of being unable to cut hard grass or small branches, and their greatest weakness is that they take an awful lot of time and wear and tear excessively.
The metal blades have varying numbers of teeth or cutting edges, ranging from two to a few tens of teeth or cutting edges. Regarding material, the metal blades are made of steel and some metal blades have carbide-tipped teeth or cutting edges. The metal blades have the advantage of being able to cut both grass and small branches sharply, thereby providing high cutting efficiency. However, if the metal blades hit an obstacle, they can get broken or damage or break the obstacle. In this way, they involve high risks and could involve a big sacrifice. Tip saw blades and the like can get nicked in an instant and are not resharpenable. Therefore, it is uneconomical to use a tip saw blade without considering where it is used.
Thus, if the two types of grass cutting blade are combined appropriately, it will be possible to carry out a grass cutting operation in a short period of time under various conditions, but so far, there has been no appropriate combination.
The present invention has been made to solve the above problem.
Conventional grass cutting blades resulting from a combination a resin string blade and metal blade include those which involve carrying a short replacement resin string (see, for example, patent document 1 or 3) and those which use a resin blade instead of a metal blade (see, for example, patent document 2).    Patent document 1: Japanese Patent Laid-Open No. 10234216 (p. 7, FIG. 16)    Patent document 2: Japanese Patent Laid-Open No. 11-235113 (p. 7, FIG. 17)    Patent document 3: Japanese Patent Laid-Open No. 2000-308407 (p. 7, FIG. 18)